Thermoforming processes typically involve vacuum forming, heat bending or folding an ink on a plastic substrate into a three-dimensional object. The temperature of the ink on the substrate is estimated to be between 60-180° C. During the thermoforming process, the ink image continuously elongates at the same rate as the plastic substrate to produce a three-dimensional product with digitally printed graphics. The printed ink image must be uniform and without cracking after the thermoforming process. US 2007/0084372 provides a general introduction to thermoforming inks. A typical thermoforming process is described in more detail below:                1) Thermoform inks are jetted onto a flat substrate and then cured with UV at an energy level of typically about 150 mJ/cm2 or EB radiation from about 15 kGy with an accelerating voltage of 100 kV.        2) The substrate is put in an oven for between 20-25 seconds at a distance of 20 cm from the IR lamps. The lamps are set at a temperature of 700° C.        3) The substrate will soften under the heat from the IR lamps and after about 25 seconds a substrate molding pattern (see FIGS. 1 and 2) comes from under the substrate and deforms it.        4) After about 40 seconds the substrate molding pattern is retracted and a jet of air is applied to cool the substrate down. The thermoforming process is then complete.        
Ink-jet imaging techniques have become very popular in commercial and consumer applications. Ink-jet printers typically operate by ejecting ink onto a receiving substrate in controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, ink-jet printers can produce a wide variety of printed features, including text, graphics, images, holograms, and the like. The present invention relates, in particular, to ink-jet inks for use as thermoforming inks that are printed onto substrates that undergoing thermoforming processes.
Thermal ink-jet printers and piezo ink-jet printers are the two main types of ink-jet systems in widespread use today. For both approaches, inks must meet stringent performance requirements in order for the inks to be appropriately jettable and for the resultant printed features to have the desired mechanical, chemical, visual, and durability characteristics.
Solvent-based and water-based jettable inks are well known. A typical water-based ink generally comprises water, a colorant, which may be a dye and/or a pigment, one or more co-solvents, and one or more additives that are included to enhance the performance of the ink. Representative examples of such additives include one or more colorants, slip modifiers, thixotropic agents, foaming agents, antifoaming agents, flow or other rheology control agents, waxes, oils, plasticizers, binders, antioxidants, fungicides, bactericides, organic and/or inorganic filler particles, leveling agents, opacifiers, antistatic agents, dispersants, and the like. Solvent-based inks include relatively volatile organic solvents. Such inks typically dry more rapidly than aqueous inks. However, such solvents may be toxic, flammable, or the like, requiring careful handling. In addition, the solvent-based inks also tend to be compatible with only a limited range of substrates.
In order to avoid using a conventional solvent, inks incorporating a polymerizable diluent have been developed. The diluent tends to function as a viscosity reducer and as a binder when cured. In the uncured state, the inks have a low viscosity and are readily jetted. However, the polymerizable diluents readily cross-link upon exposure to a suitable source of curing energy, for example ultraviolet light, electron beam energy, and/or the like, to form a cross-linked polymer network. Such inks are commonly referred to as “energy-curable” inks to distinguish them from conventional “solvent-based” inks.
Ink-jet inks are typically limited to digitally printing onto flat sheet constructions, such as papers, plastics, banner materials and the like. For three-dimensional plastic constructions, screen printing was the preferred method since digital printing of ink-jet inks onto plastic substrates produced images that cracked or could not elongate under thermal forming conditions. Therefore, there is a need to be able to digitally print directly onto plastic substrates using ink-jet printing systems to form ink images capable of continual elongation during thermal processing.
US 2006/0222831 describes a curable ink-let ink comprising mono-functional monomers, a solvent, a pigment and a diluent which is capable of adhering to plastic substrates. The ink gives continuous elongation of 100-900% during thermoforming of the substrate. Optional use of difunctional Monomers of 10,000 g/mole or more and also the use of acrylic polymers is cited in the examples.
US 2006/0275588 (Polymeric Imaging) and also WO 2007/089252 describe ink-jet ink compositions comprising a pigment, diluents, mono functional monomers and an acrylic co-polymer or polymer with an acid number of below 20 mgKOH/g. The inks can be used as a high elongation vacuum formable ink with elongation 100-900%.
US 2007/0084372 (Polymeric Imaging) describes ink compositions for a thermoforming ink with an elongation of 100-900% comprising pigment, diluents, mono functional monomers and acrylic co-polymer or polymer with molecular weights between 2500-25000 g/mol. Solvent is not used. Similarly, WO 2008/004002 (Sericol) describes the use of passive resins in thermoforming inks. Ink compositions described consist of radiation curable monomers, one or more passive resins, one or more photoinitiator and one or more coloring aids. The passive resin is present at an amount of between 2-15% and has a molecular weight 1,500-70000.
The ink-jet inks of the prior art have been found to suffer from problems with open time and/or sustainability such that they are not particularly suited for use in ink-jet printing techniques in which the nozzles of the ink-jet printer remain open between printing operations. Many known inks suffer from an unacceptably rapid increase in viscosity on being exposed to the atmosphere in a print nozzle that leads to blocking of the nozzle or poor print performance on start up. Furthermore, inks including passive resins can cause problems with jetting. Another disadvantage of many known inks is an unacceptable level of tack after cure and/or incomplete curing.
Prior art thermoformable UV-curing ink-jet inks suffer from two major problems: poor printhead start-up performance and poor printing sustainability (reliability) at higher jetting frequencies. The poor start-up performance is caused by the use of solvents or volatile UV-curing monomers in the formulation. The poor sustainability is caused by the inclusion of higher molecular weight passive resin. Passive resins are resins that are inert resins, especially inert thermoplastic resins, that do not react with the polymerisable diluent monomers during curing of the ink are known in the art. A passive resin is substantially free of functional groups which polymerise under the curing conditions to which the ink is exposed. For example, a passive resin for use in an ink that is curable in a free radical polymerisation process is a resin that is free of ethylenically unsaturated groups or other groups that participate in free radical curing reactions. Passive resins include thermoplastic acrylic resins that have a weight average molecular weight of from 1500 to 70000. They are typically included in ink formulations to reduce the degree of cross-linking required to achieve a cured film by replacing the reactive monomers and/or polymerisable diluents in an ink.